Tabular silver halide grains having a parallel twin plane (which hereinafter are referred to as tabular grains) have the following photographic properties:
(1) The ratio of the surface area to the volume (hereinafter, the ratio is referred to as specific surface area) of the tabular grains is large and a large amount of sensitizing dye(s) can be adsorbed onto the surface, whereby the spectral sensitivity is relatively higher than the inherent sensitivity thereof.
(2) When a silver halide emulsion containing the tabular grains is coated and dried, the grains are disposed in parallel to the surface of the support, whereby the thickness of the coated emulsion layer can be thinned to improve the sharpness of images.
(3) In an X-ray photographic system, by adding sensitizing dye(s) to tabular grain silver halide emulsions, the amount of crossover light can be remarkably reduced, since the extinction coefficient of the dye(s) is larger than the extinction coefficient by the indirect transition of the silver halide (AgX), whereby the degradation of image qualities can be prevented.
(4) The silver halide emulsion containing the tabular grains causes less scattering of light and hence can give images having high resolution.
(5) The silver halide emulsion containing the tabular grains has a low sensitivity for blue light and hence when the emulsion is used for a green-sensitive emulsion layer or a red-sensitive emulsion layer of a color photographic film, a yellow filter layer can be omitted from the color photographic film.
Owing to these various advantages, the tabular grain silver halide emulsions have hitherto been used for high speed photographic materials on market.
Tabular grains, of silver halide emulsions, having an aspect ratio of at least 8 (i.e., at least 8/1) are disclosed in Japanese Patent Application (OPI) Nos. 113926/83, 113927/83, 113928/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc.
The term "aspect ratio" is the ratio of the diameter of the tabular grain to the thickness thereof, and is expressed herein as ratio of two number (if omitted, the second number is considered as being 1). Furthermore, the "diameter" of the grain is defined as the diameter of a circle having an area equal to the projected area of the grain as viewed in a micrograph or an electron micrograph of an emulsion. Also, the thickness of the grain is shown by the distance between two parallel planes constituting the tabular grain.
Also, U.S. Pat. No. 4,439,520 describes a color photographic material having improved sharpness, sensitivity and graininess thereof by using tabular grains having a thickness of less than 0.3 .mu.m and a diameter of at least 0.6 .mu.m for at least one of the green-sensitive emulsion layer and the red-sensitive emulsion layer.
However, as shown in the examples of the aforesaid patent, the tabular grains prepared by the conventional process show poor monodispersibility and hence have disadvantages such as that:
(1) the increase of contrast (i.e., high gamma) of the characteristic curve cannot be expected, and
(2) when the emulsion containing the large silver halide grains and the small silver halide grains is chemically sensitized, it is difficult to apply the optimum chemical sensitization to both of the silver halide grains, because the optimum condition for the chemical sensitization differs between the large grains and the small grains.
Accordingly, various attempts for improving the monodispersibility of tabular grains have hitherto been proposed.
In the monodisperse tabular grains disclosed in Japanese patent application (OPI) No. 153428/77 and U.S. Pat. Nos. 4,150,994, 4,184,877 and 4,184,878, there is a restriction of using AgI crystals as the nuclei and the content of the hexagonal tabular silver halide grains in the present invention (which grains are thought to be a tabular grain having two parallel twin planes in one grain, and hereinafter are referred to as hexagonal tabular grains) in the grains obtained is less.
Japanese patent application (OPI) No. 142329/80 and U.S. Pat. No. 4,301,241 disclose the growing condition for monodisperse tabular grains, but since the nucleus forming condition for seed crystals used in the example was improper, the tabular grains formed contained less hexagonal tabular grains than in the case of the present invention.
Japanese patent application (OPI) No. 112142/86 discloses monodisperse twin grains, but since spherical grains are used as the seed crystals, the aspect ratio of the silver halide grains obtained in the examples thereof is up to 2.2, which is a low aspect ratio.
Japanese patent application (OPI) No. 39027/76 also discloses a similar grain forming process to the preceding Japanese patent application (OPI) but since the disclosed invention does not intend to increase the ratio of the hexagonal tabular grains of the present invention, the ratio of the hexagonal tabular grains in the disclosed grains is low.
Also, monodisperse twin grains are described in the examples of French Pat. No. 253,406, but the ratio of the projected area of triangular tabular grains measured on the micrographs of the grains described therein is at least 50%, and hence the tabular grains disclosed in the patent are different from the tabular grains of the present invention.
The silver halide emulsion containing both tabular grains having 3 or 5 parallel twin planes (triangular form) and the tabular grains having 2 parallel twin planes (hexagonal form) as described in the aforesaid patent has a demerit that the chemical sensitization thereof is not performed uniformly since the chemical sensitization characteristics of the former grains differs from that of the latter grains. Also, since the tabular grains in the aforesaid patent are formed by the application of physical ripening only after the formation of the nuclei, the tabular grains have the disadvantages that a dopant such as a metal ion, etc., cannot be introduced into a desired position in the grains under control, and also it is difficult to form large monodisperse grains.
Furthermore, when triangular tabular grains and hexagonal tabular grains each having the same projected area are compared with each other, the largest grain size portion of the former is 1.23 times the largest grain size portion of the latter and hence the graininess is improved by the existence of such a larger proportion of the hexagonal tabular grains.
Also, tabular grains are disposed in the emulsion layer in such a manner that the two parallel outer planes become parallel to the coated surface but they are randomly piled on each other in the perpendicular direction with respect to the coated surface and hence they have the disadvantages that the graininess is deteriorated and the covering power is reduced. These disadvantages are severe in the case of triangular tabular grains.